[DO NOT MERGE] Implement PoC DFA engine for nullability and truthiness

[rikkimax]

Throwing the experimental fast DFA engine at CI to see what happens, this PR is not going to be merged can be ignored.

[dlang-bot]

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#21494"

[rikkimax]

Ugh isBinExp doesn't work for EXP.loweredAssignExp flags are not set for it even though the hierarchy matches.